Conventional technologies have suggested that a barrier property of a base film can be improved to a great extent by coating the base film with an inorganic oxide thin layer. Further, a resin layer provided on the inorganic oxide thin layer imparts heat-sealing properties to the barrier film and protects the inorganic oxide thin layer.
For instance, Japanese Patent Application Laid-open No. 237940/1988 (JP-A-63-237940) discloses a composite film which comprises a metal oxide thin layer of indium oxide or tin oxide formed by sputtering, and a heat-sealing layer of an ethylene-propylene copolymer or the like formed thereon. Japanese Patent Application Laid-open No. 202435/1989 (JP-A-1-202435) and Japanese Patent Application Laid-open No. 202436/1989 (JP-A-1-202436) disclose packaging materials for microwave heating or for retort-processing in which the surface of a base film is covered with a vapour-deposited layer of silicon oxide, and either of a heat-sealing layer or a protective layer, the heat-sealing layer being formed by lamination of a polypropylene film or other heat-sealing resin films, while the protective layer being formed by lamination of a film or coating of a thermosetting resin.
However, where a polymer layer is formed by extrusion-lamination process, the barrier property of a film is terribly deteriorated because the heat and the external mechanical force generated during the process cause a crack or defect on the inorganic thin layer. A dry-lamination process for film-lamination is also affected by external mechanical force during the lamination of films or the coating of an adhesive agent, and thus is likely to damage the inorganic thin layer formed on the base film. In the case of a coating process utilised in the formation of a protective layer on an inorganic thin layer, external mechanical force instantly destroys an inorganic thin layer only to give critically decreased barrier properties, not to mention that the coating hinders the development of barrier properties. All of the above-mentioned films, including a film as produced by laminating or coating a vinylidene chloride-series resin or the like onto an inorganic oxide thin layer provided on a base film, have limited adhesive properties of the inorganic oxide thin layer to the base film. Besides, when these films are utilised as a packaging material to be subjected to external mechanical strength or as a packaging material to be exposed to a high temperature/high humidity environment, it is impossible to expect high adhesive properties and high barrier properties which can last for a long period. In particular, the adhesive properties and the barrier properties of these films decline crucially when exposed to severe conditions such as a boiling process or retort treatment process (retorting process).
Japanese Patent Application Laid-open No. 86539/1991 (JP-A-3-86539) discloses a retortable packaging film which is produced by coating the surface of a base film with a reactive two-component curable resin composition composed of a polyisocyanate compound and a saturated polyester polyol and forming a silicon oxide thin layer on the coated surface. Disclosed in Japanese Patent Application Laid-open No. 278946/1991 (JP-A-3-278946) is a packaging material for retort treatment applications which comprises an anchor coat layer, a metal oxide thin layer and a heat-sealing layer provided in this order on the surface of a base layer. Japanese Patent Application Laid-open No. 173137/1992 (JP-A-4-173137) discloses a laminate for a balloon having a good barrier property against helium and hydrogen gas, which is produced by forming an undercoat layer of a two-component curable polyurethane or the like on a film, providing an inorganic oxide thin layer on the undercoat layer, and coating the inorganic oxide thin layer with a vinyl chloride-vinyl acetate copolymer, a polyamide, a polyester or an acrylic resin, or with a hot-melt coating composition.
Japanese Patent Application Publication No. 18395/1996 (JP-B-8-18395) discloses a vapour-deposited film which comprises a base film, a mixture layer of a two-component curable polyurethane and 5 to 30% by weight of a vinyl chloride-vinyl acetate copolymer formed on the base film, and a vapour-deposited layer of a metal oxide such as magnesium oxide deposited on the mixture layer. This literature teaches the use of a reactive two-component curable polyurethane composed of a polyol component and a polyisocyanate compound for prevention of permeation of water, and the use of a vinyl chloride-vinyl acetate copolymer for enhanced adhesive properties of the vapour-deposited layer.
Japanese Patent Application Laid-open No. 131590/1993 (JP-A-5-131590) discloses a laminated film produced by depositing a silicon oxide thin layer on at least one side of a base film, thereon coating a thin layer of a silane coupling agent having an amino group in the terminal, and laminating a resin layer having an acid anhydride group on top of the layers. Japanese Patent Application Laid-open No. 239537/1991 (JP-A-3-239537) discloses a laminated film which comprises a base film, a first transparent layer of silicon oxide, a second transparent layer of a silane coupling agent, and another transparent layer of a pressure-sensitive or heat-sensitive adhesive layer laminated in this order on the base film.
These composite films can improve adhesive strength between the base film and the inorganic oxide thin layer. However, a composite film which shows high adhesive properties and barrier properties in an ordinary state (i.e. a film before subjected to retort treatment) still fails to retain good barrier properties as well as the adhesive properties between the base film and the inorganic oxide thin layer, when used as a film for retort treatment. It is impossible for these films to keep, after retort treatment as well as in an ordinary state (before retort treatment), the adhesive properties between the base film and the inorganic oxide thin layer and the barrier properties at high standards or levels. Moreover, the tackiness of the coated layer of a reactive two-component curable resin composition tends to cause blocking, making winding and rewinding of the film difficult. Thus, the productivity has to be sacrificed. Since the polyol component and the polyisocyanate compound are reactive to each other, the prepared or used two-component curable polyurethane cannot be used repeatedly and has to be discarded only to end in wasteful use of the undercoat resin composition.
To be more demanding, a packaging film for food, medical and pharmaceutical products, and precision electronic parts requires high transparency for the purpose of visual recognition of the contents and for good external appearances. In the aforesaid composite films, however, the decline of adhesive properties of the inorganic thin layer is likely to be accompanied with the decline of transparency.